Conventionally, electrophotographic image forming apparatuses such as copiers, printers, and facsimile machines have been known. The electrophotographic method makes it possible to easily form a high-quality image, and therefore has been widely adopted by the various image forming apparatuses.
Generally, an image forming apparatus adopting the electrophotographic method carries out the following operations. Specifically, a surface of a photoreceptor containing a photoconductive substance is uniformly charged. The photoreceptor's surface thus charged is irradiated with light that is based on image information, so that an electrostatic latent image is formed. The electrostatic latent image thus formed is visualized by supplying toner to the electrostatic latent image, so that a toner image is obtained. The toner image thus obtained is transferred from the photoreceptor onto a recording medium. The toner image thus transferred onto the recording medium is fixed to the recording medium by heat, pressure, and the like.
Toner to be used for developing the electrostatic latent image is supplied from a developing device to the surface of the photoreceptor. The developing device includes a developer tank. The developer tank contains toner, and has a developing roller, a doctor blade, and a stirring roller. The developing roller is provided so as to make contact with the surface of the photoreceptor, has a surface on which a toner layer is carried, and supplies toner to the electrostatic latent image. The doctor blade includes a plate member provided so as to make contact with the surface of the developing roller, and regulates the thickness of the toner layer carried on the surface of the developing roller. The stirring roller carries out a rotation operation, so that (i) the toner is charged and (ii) the toner thus charged is supplied to an area surrounding the developing roller.
The developing device further includes a toner hopper serving as a toner supplying device. The toner hopper is designed to supply toner to the developer tank.
For example, in Patent Document 1 (Japanese Unexamined Utility Model Publication No. 168458/1988 (Jitsukaisho 63-168458; published on Nov. 2, 1988)), a plurality of toner amount sensors are provided on a side wall of a supplying tank (toner hopper) so as to be on top of one another, and a supplying roller is provided in a toner-supplying opening via which toner is supplied from the supplying tank. Moreover, the rotation speed of the supplying roller is controlled in accordance with the amount of toner remaining in the supplying tank which amount is detected by the plurality of toner amount sensors.
Further, in Patent Document 2 (Japanese Unexamined Patent Publication No. 50778/1987 (Tokukaisho 62-50778; published on March 5)), a toner density sensor is provided in a developer tank, and a toner supplying roller is provided in a toner-supplying opening of a toner tank (toner hopper). Moreover, the rotation of the toner supplying roller is controlled in accordance with a result of detection carried out by the toner density sensor.
Meanwhile, in recent years, a toner supplying device including a toner cartridge has been commonly used for the purpose of increasing the level of convenience in supplying toner to a developer tank. According to such a developing device, as described in Patent Document 3 (Japanese Unexamined Patent Publication No. 255727/2001 (Tokukai 2001-255727; published on Sep. 21, 2001)) and Patent Document 4 (Japanese Unexamined Patent Publication No. 295975/1999 (Tokukaihei 11-295975; published on Oct. 29, 1999)), the toner cartridge supplies toner to a toner hopper, and the toner hopper supplies toner to the developer tank. In this case, the toner hopper functions as an adjuster by which toner supplied from the toner cartridge is smoothly supplied to the developer tank by appropriate amounts. Further, for the purpose of miniaturization of a developing device, a typical toner hopper is small, and can contain only a small amount of toner. On the other hand, for the purpose of reducing exchange frequency, a typical toner cartridge is larger than a typical toner hopper, and contains a larger amount of toner than does a typical toner hopper.
According to such a developing device, as described in Patent Document 3, a developer container (developer tank) is provided with a toner density sensor, and a toner hopper is provided with a remaining toner amount sensor. Moreover, when a shortage in toner in the toner hopper is detected by the remaining toner amount sensor, the toner cartridge supplies toner to the toner hopper. When a shortage in toner in the developer tank is detected by the toner density sensor, the toner hopper supplies toner to the developer container.
See a case where a two-step toner supplying device, in which, as described above, a toner cartridge (first toner supplying means) supplies toner to a toner hopper (second toner supplying means) and the toner hopper supplies toner to a developer tank, is arranged such that, as described above, the toner cartridge supplies toner to the toner hopper in accordance with a result of detection carried out by the remaining toner amount sensor of the toner hopper. In this case, toner may not be smoothly supplied to the developer tank. That is, slow detection of a correct remaining amount of toner or false detection of a remaining amount of toner may be caused depending on (i) a location at which the remaining toner amount sensor is disposed in the toner hopper, (ii) a direction in which toner is flowed, and (iii) the like. Alternatively, in cases where a large amount of toner is consumed for the purpose of printing a document having a high coverage rate or continuously carrying out a large amount of printing, the toner cartridge may supply toner to the toner hopper belatedly. In such a case, the density of toner contained in the developer tank becomes insufficient, and this causes a developing device to have a problem in carrying out a development operation. Such a problem becomes significant especially in an arrangement in which the toner hopper is miniaturized for the purpose of miniaturizing the device.
Therefore, it is an object of the technology described herein to provide a toner supplying method, a toner supplying device, a developing device, and an image forming apparatus each of which makes it possible to stably supply toner to a developer tank in an arrangement in which first and second toner supplying means disposed on top of each other are provided.